Blender Git Loki

Make deps: Updated required list of dependencies for macOS

It was possible to drop `nasm` from the list of required macOS dependencies. However, `pkg-config` had to be added - it was required before but probably no one noticed this.

In order to build `external_clang` successfully, `external_xml2` had to be added to the clang dependencies (`c-index-test` was failing).

Note: On Linux `make deps` still seems to require `nasm` installed via apt.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8870